Scavenge gear pump

ABSTRACT

A scavenge gear pump and its method of operation is described. The scavenge gear pump defines a pump chamber in which a pair of toothed gears are rotatably driven to pump an oil-air mixture from an inlet passage where the oil mixture is at low pressure, to an outlet passage where the air and oil has separated and is at high pressure. The gears are intermeshed in a meshing area between the inlet and the outlet passages and an oil nozzle is provided in a downstream region disposed in relation to the gear meshing area to inject oil under pressure in that area to occupy the volumes between the intermeshed gear teeth whereby to prevent air bubbles from being carried back to an upstream region adjacent the inlet passage. Accordingly, the volumetric efficiency of the pump is improved and it is more tolerant to back pressure.

TECHNICAL FIELD

The present application relates to pumps and, more particularly, to scavenge gear pumps.

BACKGROUND ART

Scavenge gear pumps are utilized in all sorts of applications whereby to pump a liquid which is received at an inlet of the pump at a low pressure and wherein the gear pump progressively increases the pressure of the fluid to a higher pressure at an outlet end. The liquid can be oil such as used in hydraulic systems or in a lubricating system such as for a gas turbine engine. Other applications of scavenge gear pumps are well known in the art. The typical scavenge gear pump may carry an air-oil mixture which is composed of about 1 to 3 volume of air for each 1 volume of oil. The oil mixture separates in the pump due to the centrifugal forces wherein the oil is released at the tooth tip of the gears while the air forms a bubble towards the gear hub. Downstream of the pump air at a higher pressure becomes trapped in a downstream area and this air enters the meshing area of the teeth of the gears and is released back at the upstream end of the pump where the air expands due to the lower pressure in that area and the air occupies space. This air build-up causes the scavenge pump to stall and re-prime itself. Also, because of this effect, the pump housings are made larger due to this air displacement between the downstream end and the upstream end of the pump. Because of this air transfer in the pump housing, the efficiency of the pump is affected as well as the volumetric efficiency thereof wherein more space is required to handle the air displacement. Obviously, the pump also does not operate at a constant capacity.

SUMMARY

According to one aspect, there is provided a scavenge gear pump comprising a pump housing having a pump chamber in which is rotatably mounted a pair of driveable toothed gears. The pump chamber has an inlet passage for receiving oil having air bubbles therein at low pressure and an outlet passage for delivering the oil at a higher pressure. The gears have radially projecting gear teeth disposed closely spaced to a respective one of opposed arcuate walls of the pump chamber to define opposed convection paths. The gear teeth of the pair of toothed gears intermesh in a gear meshing area between the gears. A downstream region is defined in the pump chamber between the outlet passage and the gear meshing area, and an upstream region is defined between the inlet passage and the gear meshing area. An oil nozzle is provided in the downstream region and is disposed in relation to the gear meshing area to inject oil under pressure in the gear meshing area and at a rate to occupy substantially all voids between the gear teeth in the gear meshing area to prevent the air bubbles in the downstream region to be convected back into the gear meshing area.

According to another aspect, there is provided a method of increasing the volumetric efficiency and back pressure of a scavenge gear pump comprising the step of injecting oil under pressure in a downstream region of the pump adjacent a gear meshing area to substantially prevent the ingress of air bubbles, present in the downstream region, into the gear meshing area whereby substantially only oil is convected by the intermeshing gears of the pump to an upstream region of the pump where an oil-air mixture enters the pump at an inlet thereof to be pressurized to the downstream region wherein a pump outlet is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures in which:

FIG. 1 is a fragmented perspective view illustrating the typical construction of a scavenge gear pump;

FIGS. 2A to 2C are cross-section views showing the operation of the typical scavenge gear pump shown in FIG. 1;

FIG. 3 is an enlarged fragmented section view illustrating how the volumetric capacity and efficiency of a scavenge gear pump is effected by air bubbles being convected from the high pressure outlet area of the pump to the inlet low pressure area of the pump;

FIG. 4A is a schematic plan view illustrating the position of the oil injection nozzle and the relief cavity in the downstream region of the pump adjacent the gear meshing area;

FIG. 4B is a cross-section view along cross-section lines A-A in FIG. 4A; and

FIGS. 5A to 5D are plan section views illustrating the displacement of the toothed gears in the gear meshing area and the transfer of oil in the voids (volumes) formed by the meshing teeth of the gears from the high pressure downstream end to the low pressure upstream end of the pump.

DETAILED DESCRIPTION

Referring now to FIGS. 1 to 3, there is shown generally at 10 a scavenge gear pump. These pumps are used for all sorts of applications for pumping hydraulic oil, for example to operate machinery implements or for use in turbine engine systems for pumping an air/oil mixture from an oil sump from a jet engine, or from an airframe or engine mounted gearboxes. The gear pump comprises a pump housing 11 having a pump chamber 12 in which is rotatably mounted, on respective shafts 13, a pair of drivable toothed gears 14 and 14′. One of the gears, herein gear 14, is a drive gear while gear 14′ which is coupled thereto is driven by the drive gear 14.

As better seen in FIGS. 2 and 3, the pump chamber 12 has an inlet passage 15 for receiving oil 16 which is herein an oil-air mixture, at low pressure and convected by the toothed gears 14 to an outlet passage 17 for delivering the oil mixture at a higher pressure. As better seen in FIG. 3, the gears 14 and 14′ have radially projecting gear teeth 18 which are disposed closely spaced to a respective one of opposed arcuate walls 19 and 19′ of the pump chamber 12 to define opposed convection paths as illustrated by arrows 20 and 20′. The gear teeth 18 of the pair of toothed gears 14 and 14′ intermesh in a gear meshing area 21 which is herein identified by stippled lines and which area is located between the gears.

A downstream region 22 is defined in the pump chamber 12 between the outlet passage 17 and the gear meshing area 21. An upstream region 23 is defined between the inlet passage 15 and the gear meshing area 21. As shown, the gear meshing area is disposed between said inlet and outlet passages.

Referring now to FIGS. 4A and 4B, there is shown an oil nozzle 25 disposed in the downstream region 22 and positioned in relation to the gear meshing area 21 whereby to inject oil under pressure in the gear meshing area 21 at a rate to occupy substantially all voids or volumes 26, see FIGS. 5B and 5C, between the gear teeth 14 and 14′ in the gear meshing area to prevent air bubbles in the downstream region 22 to be convected back into the gear meshing area 21, where these air bubbles could occupy volume and cause the pump to stall. The oil injected under pressure in this region creates a barrier to the air trapped adjacent the outlet passage 17.

As also shown in FIGS. 4A and 4B, the oil nozzle 25 communicates with a relief cavity 30 which is machined or otherwise formed in a frontal wall 31 of the pump housing in the downstream region 22 and it is configured and disposed to surround a pre-meshing section of the gear meshing area 21 to fill a volume between the intermeshing teeth in the gear meshing area.

The oil for the oil nozzle may be supplied from a gear pump outlet passage where the air bubbles have been separated from the oil, not shown, or an internal lubrication system of the gear pump, also not shown, or from a main oil pressure pump, not shown, but all of these are obvious to a person skilled in the art.

FIG. 3 illustrates a problem with these scavenge gear pumps when air is present in the oil mixture. As shown in FIG. 3, the typical scavenge gear pump carries an air-oil mixture, 1 to 3 volume of air for each 1 volume of oil. The oil mixture separates in the pump due to the centrifugal forces created by the high speed rotation of the toothed gears 14 and 14′ and the oil is released at the tooth tips of the teeth 14′″ while the air forms a bubble or bubbles which are directed towards the gear hub 27. As hereinshown, the air bubble 28 entering the pump chamber 12 is at low pressure and as the gears rotate, the pressure increases by the forces applied by the speed of rotation of the gears and the air bubble is compressed, as shown by bubbles 28′, as it is carried to the downstream region 22. Because the gears are operating in counter-rotation as shown by arrows 29 and 29′, often these compressed air bubbles, such as the air bubble 28″, become trapped in the volumes or spaces 26 in the gear meshing area 21 and are carried back to the upstream area 23 of the pump, thus occupying space in that area as the air volume of the bubble expands in that area due to its transfer from a high pressure area to a low pressure area. Thus, the efficiency of the pump is reduced due to the air build-up in the upstream area 23 of the pump.

For the typical scavenge gear pump, the gear geometry creates gear meshing volumes of which is approximately 15%. An ideal pump with zero leakage will stop transferring any volume at a pressure ratio of less than 7 as the typical 15% volume of air returned from the downstream region expands to 105% upstream thus reversing the flow through the pump. At a pressure ratio of 4.5, the effective pump displacement is only 32.5%=100%−4.5×15%. The oil flow rate in the nozzle 25 amounts to 15% of the pump capacity in order to fill the volumes 26 between the intermeshing teeth. This oil is carried to the upstream region 23 of the pump and retains the same volume which is incompressible. The ideal zero leakage pump will pump against any adverse pressure ratio as the return oil volume does not change with the pressure and the effective pump capacity remains 100% at any pressure ratio. The leakages in an actual pump will therefore limit the maximum back pressure. However, the oil injected in the gear meshing area does not add to the existing pump leakage as it feeds the same leakage paths. The advantage of injecting oil in the high pressure downstream region is to prevent the pump to stall and re-prime itself due to this air transfer. Because the volumetric capacity is unaffected by back pressure, this now allows for the design of smaller pumps due to the fact that we do not have to account for air being returned to the low pressure side of the pump. Also, the oil tank is better pressurized as well as the installation of coolers in the scavenge line becomes possible without resorting to special cooler designs.

In summary, the present application also teaches a method of increasing the volumetric efficiency and back pressure of a scavenge gear pump by injecting oil under pressure in a downstream region of the pump adjacent the gear meshing area to substantially prevent the ingress of air bubbles in that area. Therefore, only oil is convected by the intermeshing gears of the pump to an upstream region of the pump where an oil-air mixture enters the pump at an inlet thereof to be pressurized to the downstream region where the pump outlet is provided.

The provision of the nozzle 25 provides for a scavenge gear pump having increased volumetric efficiency and which is more tolerant to back pressure. The pump can operate at increased volumetric efficiency and increased back pressure. The size of the pump can be reduced by substantially eliminating the transfer of air bubbles between the downstream region to the upstream region of the pump.

The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiment described therein without departing from the scope of the disclosure. It is therefore intended to cover any obvious modifications provided that these modifications fall within the scope of the appended claims. 

What is claimed is:
 1. A scavenge gear pump comprising a pump housing having a pump chamber in which is rotatably mounted a pair of driveable toothed gears, said pump chamber having an inlet passage for receiving oil having air bubbles therein at low pressure and an outlet passage for delivering said oil at a higher pressure, said gears having radially projecting gear teeth disposed closely spaced to a respective one of opposed arcuate walls of said pump chamber to define opposed convection paths, said gear teeth of said pair of toothed gears intermeshing in a gear meshing area between said gears, a downstream region defined in said pump chamber between said outlet passage and said gear meshing area and an upstream region defined between said inlet passage and said gear meshing area, and an oil nozzle in said downstream region disposed in relation to said gear meshing area to inject oil under pressure in said gear meshing area at a rate to occupy substantially all voids between said gear teeth in said gear meshing area to prevent air bubbles in said downstream region to be convected back into said gear meshing area.
 2. A scavenge gear pump as claimed in claim 1 wherein said oil nozzle communicates with a relief cavity formed in a wall of said pump housing in said downstream region and disposed to surround a pre-meshing section of said gear meshing area to fill volumes between said intermeshing teeth in said gear meshing area.
 3. A scavenge gear pump as claimed in claim 2 wherein said oil nozzle is supplied oil from one of said gear pump outlet passage where said air bubbles have been separated from said oil, from an internal lubrication system of said gear pump or from a main oil pressure pump.
 4. A scavenge gear pump as claimed in claim 1 wherein said oil nozzle injects oil at a flow rate of about 15% of the capacity of said gear pump to fill the volume between said intermeshing gear teeth in said gear meshing area.
 5. A scavenge gear pump as claimed in claim 1 wherein said one of said toothed gear is a drive gear, the other of said toothed gears being driven by said drive gear in toothed engagement therewith, said gear meshing area being aligned between said inlet and outlet passages.
 6. A scavenge gear pump as claimed in claim 1 wherein said scavenge gear pump is an oil pump associated with a turbine engine system for pumping an air/oil mixture from an oil pump of a jet engine, or from an airframe or engine mounted gearboxes.
 7. A method of increasing the volumetric efficiency and back pressure of a gear pump comprising the step of injecting oil under pressure in a downstream region of said pump adjacent a gear meshing area to substantially prevent the ingress of air bubbles, present in said downstream region, into said gear meshing area whereby substantially only oil is convected by intermeshing gears of said pump to an upstream region of said pump where an oil-air mixture enters said pump at an inlet thereof to be pressurized to said downstream region wherein a pump outlet is provided.
 8. A method as claimed in claim 7 wherein said scavenge gear pump defines a pump housing having a pump chamber in which is rotatably mounted a pair of drivable toothed gears, said pump chamber having an inlet passage for receiving an air-oil mixture at a low pressure and an outlet passage for delivering said oil at a higher pressure, said gears having radially projecting gear teeth disposed closely spaced to a respective one of opposed arcuate walls of said pump chamber to define opposed convention paths, said gear teeth of said pair of toothed gears intermeshing to define said gear meshing area disposed between said inlet passage and said outlet passage, said method further comprising driving said gears wherein said air-oil mixture entering said inlet passage is mixed with said oil convected by said intermeshing gears and entrained in said opposed convention paths wherein said oil and air therein is compressed, at least some of said compressed air being captivated in a downstream region between said outlet passage and said gear meshing area and prevented from migrating to said gear meshing area by said injected oil under pressure.
 9. A method as claimed in claim 8 wherein there is further provided the step of forming a relief cavity in a wall of said pump housing in said downstream region and disposed to surround a pre-meshing section of said gear meshing area, forming an oil nozzle in communication with said relief cavity, said oil under pressure being injected through said oil nozzle into said relief cavity.
 10. A method as claimed in claim 9 wherein said oil injected into said relief cavity amounts to about 15% of the capacity of said gear pump whereby to fill volumes between said intermeshing gear teeth in said gear meshing area. 